Recently, a need exists to promote planned performance of steps against global warming. Of the total greenhouse effect gases discharged in Japan, energy-derived CO2 emissions account for about 90% in recent years. Moreover, of the total power generated, coal-fired power plant discharges 50% CO2. Thus, coal-fired power plants are required to promote the use of new types of energy having low environmental impact.
Organic substances repeat a cycle of decomposition, absorption, and release. Equilibrium can thus be achieved for the amount of CO2 discharged by biomass energy by having a source of absorbing the equal amount of CO2. In this way, biomass is a carbon neutral fuel and thus biomass power generation carries the weight of expectations as new energy, which can reduce the amount of used fossil fuels and the amount of CO2 emissions. Easily collectable woody biomass includes wood pellets and wood chips.
Additionally, use of the biomass fuel as auxiliary fuel in the coal-fired boiler reduces the amount of NOx contained in combustion exhaust gases because the biomass fuel contains a low nitrogen content.
Against this background, coal-fired boilers are required to introduce the biomass-mixed combustion system so as to promote the use of new energy.
Among boilers using biomass, there is a biomass-mixed fired boiler that burns pulverized fuel which is a mixture of pulverized coal and biomass fuel. A typical system uses a conventional pulverized coal-fired boiler and manufactures a mixed fuel of pulverized coal and biomass by, for example, adding a woody biomass material to a roller or other type of mill that crushes coal into fine powder. The system then conveys the mixed fuel on conveyance air and burns the mixed fuel using the pulverized coal burner.
The roller mill pulverizes coal into fine particles of commonly 200 μm or less, preferably about 70 μm, in order to improve combustion efficiency of the burner. At this time the coal and the biomass fuel are treated together, thus the biomass fuel is also pulverized into fine particles. The produced mixed fuel has an aggravated product grain size results with a resultant increase in the amount of coarse components of 100 μm or more. The product fuel has a grain size distribution expanded both in coarse and fine directions. Further, fine pulverization of the biomass fuel requires a great power, which increases the unit requirement.
In addition, the woody biomass fuel and the pulverized coal have combustion characteristics different from each other. For example, the woody biomass has a volatile content twice as high as that of coal. The wood pellet has a calorific value of ⅔ of that of coal and the wood chip has a calorific value of ½ of that of coal. The wood pellet and the wood chip have an ash content of 1/10 or less of that of coal. Meanwhile, the woody biomass fuel and the pulverized coal require different amount of air for combustion. Thus, when the woody biomass and the pulverized coal are co-fired with certain amount of air, depending on the mixing ratio of the woody biomass and the pulverized coal, the combustion condition will not always be preferable. The biomass fuel mixing ratio (calorific value ratio) in the pulverized coal boiler is 3% in terms of actual industrial applications and the limit is estimated to be about 5%.
To burn the woody biomass fuel efficiently, a biomass fired burner can be employed so that the pulverized coal and the woody biomass fuel will be burned separately.
The finer the woody biomass is pulverized, the more the power is required in pulverization, which increases the unit requirement. On the other hand, the woody biomass fuel is easier to burn than coal if particle diameters are the same, which eliminates the need for making small the pulverized grain size.
When a biomass fired burner is used together with a pulverized coal-fired burner, a pulverizing mill can be operated under conditions suitable for the woody biomass fuel independently of the pulverized coal. A boiler can be operated with a suitable mixed fuel burning ratio selected as against the pulverized coal fuel.
Patent Document 1 discloses a biomass fired burner that is applied to a biomass-mixed fired boiler that loads pulverized coal and woody biomass fuel through respective lines into a furnace for combustion. The disclosed biomass fired burner includes a biomass fuel jet nozzle. The biomass fuel jet nozzle includes: a disperser disposed at a center of the biomass fuel jet nozzle, the disperser preventing uneven flow of the biomass fuel; a venturi disposed upstream inside the nozzle, the venturi increasing flow velocity of the fuel to thereby cause biomass fuel particles to collide with the disperser; a flame stabilizer disposed at a leading end of the biomass fuel jet nozzle, the flame stabilizer having a stepped enlarging structure for sharply expanding the biomass fuel stream; and a combustion air nozzle disposed on the outside of the biomass fuel jet nozzle, the combustion air nozzle supplying a secondary air swirl flow.
The biomass fired burner is optimized for burning a predetermined amount of biomass fuel. The number of biomass fired burners to be installed may be determined according to the amount of biomass fuel to be processed required in the furnace to which the burners are applied. The arrangement disclosed in Patent Document 1 has a mixed fuel burning ratio of 15%.
Patent Document 2 discloses a boiler that includes a biomass-mixed fired burner burning pulverized coal and biomass fuel and a boiler that includes a starting or auxiliary burner that functions also as a biomass fuel burning burner that burns biomass fuel supplied intermittently thereto. Patent Document 2 does not, however, describe any specific configuration of the biomass fired burner, problems encountered during its use, solving means, and the like.
Patent Document 3 discloses a pulverized coal fired burner. The disclosed burner is adapted to pulverized coal that has a greater calorific value, a greater amount of air required for combustion, and greater specific gravity than those of the biomass fuel and thus has a small optimum grain size. Accordingly, the burner in Patent Document 3 cannot be directly used for burning the woody biomass fuel.